1. Field of the Invention
The present invention relates to hydraulic suppressors and, more particularly, to quick response hydraulic suppressors for piping systems, which suppressors resist pipe motion resulting from oscillatory or shock loads while allowing the slow movement resulting from thermal expansion and contraction.
2. Description of the Prior Art
Shock suppressors are velocity or acceleration sensitive devices which allow for the free thermal movement of piping systems or equipment but restrain movement that results from extremely high or oscillatory loads that may occur during emergency situations such as accidents, earthquakes or pipe ruptures. Such devices are commonly used in power plants and, in particular, nuclear power plants where all possible safety precautions are required. Hydraulic devices in general have been used for shock suppression; however, in recent years, the hydraulic devices have been losing acceptability in the nuclear industry due to field maintenance problems. Thermal aging of elastomeric hydraulic seals has necessitated a five-year seal replacement program, during scheduled power plant shutdowns. In addition, the temperature ranges to which such devices may be subjected has also been limited.
In an attempt to overcome the problems associated with hydraulic devices, mechanical shock suppressors have been developed as a replacement for hydraulic devices; however, these devices have also experienced problems with inconsistent operation and jamming.
A number of hydraulic and mechanical devices and combinations thereof have been proposed with varying degrees of success. U.S. Pat. No. 3,148,852 issued to E. D. Lord et al. shows a very basic fixed orifice suppressor which provides for concurrent seismic and thermal movement by the flow of fluid through the fixed orifices 26. This device does not provide a sharp break in the operational characteristic curve so that a low resistance is provided for thermal movement and a high resistance for shock movement, but rather provides increasing restraint for increasing force and would not be satisfactory for the quick response applications envisioned for the present invention.
Canadian Pat. No. 667,228 issued to P. C. Sherburne shows a hydraulic suppressor that utilizes a fixed orifice in parallel with a check valve wherein the check valve opens at a predetermined threshold pressure to protect the suppressor from overpressure conditions. U.S. Pat. No. 2,838,140 issued to M. B. Rasmusson et al and U.S. Pat. No. 3,827,537 issued to H. E. Haller, Jr. et al. each show the use of a check valve in parallel with an orifice for controlling fluid flow in a hydraulic cylinder. In these patents, the orifice is controlled by a needle valve and in the Haller, Jr. et al. patent, a more sophisticated embodiment shown in FIG. 9 discloses the use of a poppet-type valve that closes upon the application of a predetermined pressure.
U.S. Pat. No. 3,739,808 issued to L. R. Landherr shows a hydraulic suppressor utilizing a pressure-responsive spool valve which closes under a predetermined pressure to cut off the hydraulic fluid flow and thereby suppresses further motion. U.S. Pat. Nos. 3,561,574 and 3,572,363 issued to H. R. Dickinson, Jr. et al. and D. E. Roach show devices utilizing series connections of fixed orifices, one orifice being smaller than the other for controlling fluid flow. The smaller orifice is normally bypassed to allow free movement resulting from thermal expansion and contraction and minor shocks. When large shocks above a predetermined level are sensed, the bypass around the small orifice is closed forcing the hydraulic fluid through the small orifice, thereby suppressing the larger shocks.
U.S. Pat. No. 3,547,236 issued to G. Leisegang, U.S. Pat. No. 3,827,537 issued to H. E. Haller, H. F. Huettner and E. E. Martin Jr., and U.S. Pat. Nos. 3,106,992 and 3,702,646 both issued to P. C. Sherburne disclose the use of pressure-responsive valves in conjunction with fixed bypass orifices in hydraulic suppressors. The Leisegang and Sherburne '646 patents show the use of poppet-type valves which normally remain open to allow a free flow of fluid to and from the cylinder but close upon the sensing of a predetermined pressure, thereby restricting movement of the piston and the cylinder. On closure of the poppet-type valves, a bleed orifice is provided to permit some continued motion and to vent the pressure behind the poppet valve to allow a subsequent opening after the pressure is reduced. Considering this type of device, which is the most commonly used in the systems today, it is apparent that the suppressor is activated as a shock suppressor only when the piping system reaches a predetermined actuation velocity that is greater than the normal thermal movement of piping. If conditions immediately subsequent to a shock event cause thermal movement of the piping system, a lock-up condition could result. Since the bleed orifice is not of sufficient size to provide for both thermal movement and bleed-off of high pressure fluid, the poppet valve remains closed after a shock condition due to the unrelieved pressure. As a result, the suppressor remains in a checked or resistive mode after the shock event has passed, and the continued thermal movement induces high stresses in the piping system. This condition could cause a tentative failure or overstressed load condition on the piping system.
FIG. 1 illustrates the phenomenon that occurs when a poppet valve remains closed after a seismic event that is followed by continued thermal motion. The lower curve shows displacement in inches versus time and the upper curve shows the load exerted on the piping system versus time. A dynamic force at a frequency of 5 hertz and at rated load is applied to simulate a seismatic event, then a constant motion of 4 inches per minute is applied to the system to simulate thermal movement. A review of the curves will reveal that during the first second, only the seismatic load was exerted on the system and the resulting limited oscillatory motion is illustrated. Subsequent to the application of the oscillating load, thermal movement in the form of a constant velocity input was imposed and it is seen how the displacement shifted linearly at the rate of the constant velocity input. The oscillating load was then removed and only the constant velocity thermal movement continued; however, it may be noted that the load continued to increase up to and beyond the rated load of the suppressor, placing undue stress on the piping system due to the suppressors remaining in a locked or rigid condition. This is a major problem towards which the present invention is directed.